This invention relates generally to rotary machines and more particularly, to methods and apparatus for controlling wind turbine generator air gap dimensions.
Generally, a wind turbine generator includes a rotor having multiple blades. The rotor is sometimes mounted within a housing, or nacelle, that is positioned on top of a base, for example a truss or tubular tower. At least some known utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have rotors of 30 meters (m) (98 feet (ft)) or more in diameter. The rotor blades transform mechanical wind energy into a mechanical rotational torque that drives one or more generators. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into the utility grid. Gearless direct drive wind turbine generators also exist.
In the generator, rotor components and stator components are separated by a clearance gap, sometimes referred to as an air gap. A uniform air gap facilitates operation of the generator. During operation, a magnetic field generated by a plurality of permanent magnets or wound coil magnets mounted on the rotor or stator passes through a portion of the air gap defined between the rotor and the stator. A plurality of forces that are at least partially proportional to the magnitude and direction of the magnetic field are induced. These induced forces include, but are not limited to, radial and axial forces across the air gap, and torque forces, such that a plurality of forces are acting on the rotor. Transmission of the magnetic field through the air gap may be at least partly dependent on a magnitude of each of the induced magnetomotive forces (MMF) and a predetermined magnitude of an air gap radial dimension, i.e., the radial distance between a rotor surface and a stator surface. However, asymmetric and/or transient loads on the rotor may be introduced via the blades and/or other mechanisms. Such asymmetric and/or transient loads may sometimes deflect the rotor such that the air gap dimension is reduced and/or altered to be non-uniform.